Patent GB-1452772-A (GB)
Title: Apparauts for controlling centrifugal compressors
IPC: H02H7/08
Filed: 1973-09-20
Abstract: 1452772 Control of A.C. motors OWENSILLINOIS Inc 20 Sept 1973 44590/74 Divided out of 1,452,771 Heading H2J A control system for sustaining the operation of an electric-motor driven centrifugal compressor through short-duration interruptions of electric power supplied to the motor comprises a switch means to connect the compressor motor to a source of electric power; an auxiliary control circuit controlling the swich means and connectible to the source to be energizable thereby to close the switch means; and a main control circuit connectible to the source and operable both to maintain the auxiliary control circuit energizable for a first preselected time period after a complete or partial loss of electric power, thereby enabling restoration of the power during the first preselected period to re-energize the auxiliary control circuit, and to prevent the reenergization of the auxiliary control circuit by restoration of the power in the event of either: (a) a further complete or partial loss of power within a second preselected time period which is of greater duration than the first preselected time period and commences at the end of the last complete or partial loss of power during the first preselected time period, or (b) the absence of full restoration of power within the first preselected time period. Figure 1 shows a control system for the motor driving a centrifugal compressor, the motor/compressor combination being started and controlled as described in copending Specification 1,452,771. The auxiliary control circuit consists of: (1) a safety interlock circuit comprising a reset switch 14, locking contacts 3CR1, safety interlock elements 16, 18, 20, 22, ICR1 and 2TR1 and a relay coil 3CR, and (2) an enabling circuit comprising contacts 3CR2, start switch 28, locking contacts 4CR1, limit switches 30, 31, a minimum oil temperature switch 32 and a relay coil 4CR. The main control circuit consists of: (1) a safety interlock restart circuit comprising contacts 2CR1 and 6TR2; (2) an enabling restart circuit comprising contacts 7TR1 and 6TR1; (3) a power dip timing circuit comprising contacts 4CR3, 5TR1, 5CR1, 6TR3, 5CR2 and 9TR1 and relay coils 5TR, 5CR, 6TR, 7TR and 9TR, and (4) a redundant, fail-safe, restart circuit comprising relay coils 1TR, 2CR, 2TR ' and contacts 1TR1. The switch means consists of a contactor (MR1, MR2, MR3), Fig. 3 (not shown), whose operating coil (MR) is controlled by the contacts (4CR2) of relay 4CR, this series combination being shunted by a relay coil (1CR) controlling the contacts 1CR1. Relays 6TR, 7TR have an opening time delay constituting the first preselected time period, whereby contacts 6TR2 and 6TR1, 7TR1 remain closed during this period to permit the reenergization of relays 3CR, 4CR following one or more power failures and restorations occurring during this period. During such failures contacts 2CR2, 2CR3 remain closed and contacts 5CR2 open, whereby coils 2SV, 3SV and 4SV controlling valves (44, 38, 42) in the compressor control system, Fig. 2 (not shown), maintain two of these valves closed and open the third so that operation continues in the condition existing just prior to the failure. Relay 9TR has a closing time delay which is longer than the opening delay or relay 7TR and constitutes the second preselected time period, whereby the contacts 7TR1 open before contacts 9TR1 close following the last restoration during the first time period. Hence the contacts 7TR1 will be open for a failure and restoration which occurs during the second period and is either completely or partially outside the first period, whereby the relay 4CR cannot be re-energized and the compressor is shut down. If such a failure does not occur, the contacts 9TR1 close to reset the power dip timing circuit for its next operation. Restoration of the supply during a permitted re-energization condition energizes relay 2CR through contacts 1TR1 which open after a time delay, failure to do so causing energization of relay 2TR and opening of its contacts 2TR1 to shut down the compressor. Also described are: (1) a complete start-up operation of the compressor, relay 5TR having a closing time delay whereby the motor reaches full unloaded speed before control commences of valves (46, 56) in the compressor system, Fig. 2 (not shown); (2) the function of protective elements 18, 20, 22, 32 and limit switches 30, 31 responsive to valve positions, and (3) the function of relay 2CR, which has contacts (2CR4, 2CR5) to shunt signals representative of motor current during start-up or restart after a short duration power failure, Fig. 2 (not shown).